1. Field of the Invention
This invention relates to holdup time provided by a power supply and more particularly relates to preserving energy in a power supply that has lost input power in order to maximize system holdup time.
2. Description of the Related Art
Computer systems use power supplies to convert power from one form to another. Other electronic systems use power supplies as well. Typically, a computer system requires that a power supply convert an alternating current (“AC”) input voltage to a direct current (“DC”) voltage that may be used for internal computer components. In other applications, a power supply converts an AC voltage to a DC voltage, and then back to an AC voltage.
FIG. 1 is a schematic diagram 100 of a computer 102 with two AC-to-DC power supplies, power supply 1 104 and power supply 2 106, connected to a bus 108. The diagram 100 shows typical components of a power supply in power supply 1 104. Typically, an input power 109 is applied and connects to a connection point. The connection point may be input terminals and is depicted here as AC input terminal 112. Typical input power voltages are 120 volts alternating current (“VAC”) single phase, 208 VAC single phase, 208 VAC three phase, and 408 VAC three phase. Other voltages are typical in specialty and foreign power systems. The input power 109 may also be in the form of a DC voltage. DC input voltages are typical in systems such as telephone equipment. The AC input terminal 112 may also include other power conditioning components such as surge suppressors.
Typically a power supply converts a higher AC input voltage to a lower DC voltage. For example, a power supply in a personal computer may convert 120 VAC single phase to 12 VDC. Other output voltages are common such as 5 VDC or 3.3 VDC. Typically, a computer 102 will include a bus, or other distribution network, for each voltage used in the computer 102. A converter with an AC input and a DC output is typically called an AC-to-DC converter. A converter with a DC input and a DC output is typically called a DC-to-DC converter.
Power supply 1 104 may also include an electromagnetic interference (“EMI”) filter 114. The EMI filter 114 filters out unwanted frequencies generated in the power supply 1 104 or traveling on the input power 109 connection. For AC systems, the power supply 1 104 includes some type of rectifier 116. For example, the rectifier 116 may be a half bridge or a full bridge rectifier constructed with diodes or components configured to be diodes. The rectifier 116 typically converts AC voltage to DC voltage. The rectifier 116 may include a capacitor after the diode components of the rectifier.
The power supply 1 104 may also include an inrush current limiter 118 to limit current when power is first applied to the power supply 1 104. A current limiter maybe a choke or other coupled magnetic element configured to limit current inrush.
The power supply 1 104 may also include a power factor correction (“PFC”) booster 120. A power factor (“PF”) control 122 circuit may also be included. The PFC booster 120 and PF control 122 are typically configured to compensate for low power factor generated by a power supply, such as power supply 1 104. The PFC booster 120 and PF control 122 may typically use active components and switching. The PFC booster 120 and PF control 122 are typically configured to create a current as seen at the AC input terminal 112 such that the power factor is high.
Note that many power supply configurations are possible and that many of the above mentioned elements may not be included in a power supply. For example, a DC-to-DC power supply may not include the rectifier 116. Less expensive power supplies or power supplies in non-sensitive environments may not include an EMI filter 114. While there is typically some means to connect the input power 109, the AC input terminal 112 may not include surge suppressors, terminals or other conditioning elements. Many power supplies are configured without power factor correction so the PFC booster 120 and PF control 122 may not be included. Inrush current may not be a problem due to natural impedances so an inrush current limiter 118 may not be included.
In situations where higher reliability is desired, multiple power supplies may be connected to a common bus. For example, power supply 1 104 connected to a bus 108 may derive input power 109 from one power system and power supply 2 106 also connected to the bus 108 may derive power from another power system. Such a system would have an increased reliability over a single power supply system because loss of power in one power system would only affect the power supply connected to the power system that lost power.
When a computer system loses power, data may be lost. One way to minimize data loss is to sense when power is about to be lost and then store pertinent data before the system actually loses power. The time between when a power loss is sensed and when voltage levels on a computer bus or a power supply feeding the bus drops to an unusable level is often called holdup time. Typically holdup time is on the order of hundreds of milliseconds. Batteries may be used in some applications to provide power when input power is lost. In many situations, however, batteries may not be able to provide power at full amperage as may be required to save critical data.
A power supply may be used to create a holdup time. A power supply may provide full power or near full power for a short period of time. Typically, a power supply includes one or more energy storage components such as a capacitor. If a minimum holdup time is required, a capacitor or capacitors in a power supply may be sized to store enough energy to maintain the computer bus above a minimum voltage level. A capacitor may be sized for holdup and may be larger than what may be required for normal converter operation in a power supply.
Power supply 1 104 includes a bulk capacitor 124 used for holdup time. The bulk capacitor 124 may comprise a capacitor bank. The bulk capacitor 124 may also be used for voltage and current smoothing of the rectified waveforms from the rectifier 116 or other components. Power supply 1 104 also includes a DC-to-DC converter 126 configured to convert the DC voltage at the bulk capacitor 124 to the voltage to be used on the bus 108. The DC-to-DC converter 126 is controlled typically with a feedback loop, here shown as a sense and control circuit 128, sensing the voltage on the bus 108 and comparing it to a reference voltage.
The voltage on the bulk capacitor 124 is typically higher than the voltage of the bus 108 so that some form of buck converter may be used to convert the higher bulk capacitor 124 voltage to the lower bus 108 voltage. Since a converter may have a wide input voltage range, if input power 109 is lost or some other component fails ahead of the bulk capacitor 124, the energy stored in the bulk capacitor 124 will temporarily allow the DC-to-DC converter 126 to continue to operate. The operation will continue until the voltage on the bulk capacitor 124 is reduced to a level where the DC-to-DC converter 126 can no longer sustain the bus 108 voltage. The time from when input power 109 loss is detected or a component failure stopping power transfer to the bulk capacitor 124 to a time when the bus 108 voltage drops below a minimum level is typically called holdup time. As the value of the bulk capacitor 124 is increased, the holdup time is increased.
If the output voltage of the power supply 1 104 drops below the voltage of the bus 108, current will flow to the power supply 1 104 which may drag down the voltage of the bus 108. Typically, when multiple power supplies 104, 106 are connected to a bus 108, the power supplies 104,106 are configured so that failure of a single power supply 1 104 will not affect the bus 108. One way to protect a common bus 108 is to use a diode function at the output of each power supply. Power supply 1 104 includes a power diode 130 which may be a diode or another electronic component, such as a metal-oxide semiconductor field-effect transistor (“MOSFET”) configured to operate as a diode. Typically, the electronic component is connected between the DC-to-DC converter 126 output and the bus 108 and is configured to protect the bus 108 when the DC-to-DC converter 126 output voltage suddenly drops below the voltage of the bus 108. If power supply 1 104 loses power or a component ahead of the bulk capacitor 124 fails, the voltage of the DC-to-DC converter 126 will drop. As the output voltage drops, the electronic component configured as a diode (power diode 130) becomes reverse biased so that current will substantially not flow back into power supply 1 104 and will not drag down the bus 108 voltage. Once the diode configured component 130 is reverse biased, other power supplies 106 connected to the bus 108 maintain the bus voltage.
Currently, systems with more than one power supply 104,106 connected to a bus 108 allow a power supply 104 that can no longer deliver power to the bus 108 to deliver power until the output voltage drops low enough for the diode configured component (the power diode 130) connected to the bus 108 to be reverse biased. Once the energy stored in the failed power supply 1 104 is delivered to the bus 108, the energy is no longer available for holdup time. The remaining power supply 2 106 is left to provide the required holdup time. If the remaining power supply 2 106 fails in a way that it cannot provide power to the bus 108, such as the diode component of power supply 2 106 failing open, power supply 2 106 cannot fulfill the holdup requirement of the computer 102.
Power supply 1 104 and power supply 2 106 may each include an early power off warning (EPOW) control 132 or similar module. In current systems, the EPOW control 132 sends a signal to the computer 102 warning that power will be lost in the power supply 104, 106. EPOW control 132 in power supply 1 104 may communicate with power supply 2 106. In one embodiment, when power is lost in both power supply 1 104 and power supply 2 106, the computer 102 takes action to prevent data loss during any available holdup time. In currently available systems, when power supply 1 104 has lost input power 109 and power supply 2 106 becomes unable to deliver power to the bus 108, holdup time is unavailable to the computer 102.
From the foregoing discussion, it should be apparent that a need exists for an apparatus, system, and method to maximize power system holdup time. Beneficially, such an apparatus, system, and method would maintain energy for holdup time in a power supply that can no longer deliver power to a bus for use if another power supply is unable to provide holdup power. The apparatus, system and method would increase system reliability by providing redundant holdup time even after a power supply is unable to deliver power.